Re: [boost] [serialization] fast array serialization (10x speedup)

From: Matthias Troyer (troyer_at_[hidden])
Date: 2005-10-10 07:31:52

On Oct 10, 2005, at 2:09 PM, Martin Slater wrote:

>>
>> then I see several major disadvantages of this approach:
>>
>> 1.) it fixes the value types for which fast array serialization can
>> be done
>>
>
> I worked around this by introducing an intermediary type
> ArchiveByteArray as follows

> ...
>      struct BinaryArchiveByteArrayO
>      {
>          int count;
>          const void *ptr;
>      };
>
>      template<class T>
>      BinaryArchiveByteArrayO MakeArchiveOutputByteArray(int count,  
> const
> T *t)
>      {
>          BinaryArchiveByteArrayO res;
>
>          res.count = count * sizeof(T);
>          res.ptr   = t;
>
>          return res;
>      }

There is a problem here: the type information gets lost once you  
create an BinaryArchiveByteArrayO, but the implementation of save 
(BinaryArchiveByteArrayO const&) will require that information in the  
case of XDR, MPI and other archives.

>    ...
>
>      template<class U, class Allocator>
>      inline void save(
>          BinaryOutputArchive& ar,
>          const std::vector<U, Allocator> &t,
>          const unsigned int /* file_version */
>      ){
>          boost::mpl::if_<boost::is_pod<U>, Detail::VectorSavePodImp,
> Detail::VectorSaveImp>::type(ar, t);
>      }

This is similar to the way I propose to implement vector  
serialization. Just look at the diffs I attached to my mail. The main  
difference is that instead of the mpl::if_ I use boost::enable_if.

> This could probably have a dispatch mechanism above it checking for an
> archive trait to dispatch to either the fast or default implemention.

Exactly, a trait will be more flexible here, since your dispatch  
based on boost::is_pod<U> might be too narrow for some archives, and  
too broad for others (some such as XML archives might never want to  
support it).

Matthias